1. Field of the Invention
The present invention relates to an electrophotographic developing unit for use in a copying machine, printer, facsimile and the like.
2. Description of the Related Art
Referring to FIG. 15, there is shown an essential part of a conventional image forming apparatus such as a printer, the apparatus comprises a photoreceptor drum 101 which is rotatable about its center axis in a direction indicated by an arrow and provided with a fixed magnet 102 therein. A corona charger 103 with a grid electrode 104 is arranged adjacent to the drum 101 at a predetermined position. The corona charger 103 charges the surface of the drum 101 at, for example -500 V. A laser beam 105 representing an image to be reproduced is radiated on the charged surface of the drum 101 so that an electrostatic latent image is formed on the surface of the drum 101. The electric potential at the latent image region exposed to the laser beam becomes, for example -100 V. A developing unit D is slantingly disposed below the passage of the laser beam 105. The developing unit D has a housing as a toner reservoir which is divided into three chambers 106, 114a and 114b. The chambers 106, 114a and 114b are, respectively, called here "a toner applying chamber, a first toner replenishing chamber and a second toner replenishing chamber". An electrode roller 107 including a fixed magnet therein is arranged in the toner applying chamber 106 such that a cylindrical surface of the roller 107 is close to the rotating surface of the drum 101. The roller 107 is rotatable in a direction indicated by an arrow. An electric voltage is applied on the roller from the alternating current/high voltage source 111. When the surface of the drum charged and radiated rotationally passes through the toner applying chamber 106, the toner is deposited on the surface in the form of magnetic brush. Thereafter, when the surface of the drum with the toner passes the roller 107, on which alternating voltage 750 V0-p superimposed by the direct current voltage -300 V is applied from the power source 111 so that the deposited toner 113 on the non-image region of the surface of the drum 101 is transferred onto the surface of the roller 107, while the toner forming the image remains on the surface of the drum 101. The transferred toner on the roller 107 is removed by a scraper 112 secured on the partition wall between the chambers 106 and 114a. The toner on the drum 101 corresponding to the image to be reproduced is transferred onto the copy sheet S supplied between the drum 101 and a transfer roller 118. Thereafter, the toner image on the copy sheet is fixed by passing the copy sheet through a fixing section (not shown).
As described above, the chambers 106 and 114a are separated by a first partition wall, and the first and second chambers 114a and 114b are separated by a second partition wall. The first and second partition walls have, respectively, toner supplying openings 115a and 115b. The toner accommodated in the second toner replenishing chamber 114b is transported through the second opening 115b into the first toner replenishing chamber 114a. Similarly, the toner accommodated in the first toner replenishing chamber 114a is transported through the first opening 115a into the toner applying chamber 106.
In the toner replenishing chambers 114a and 114b, there are provided rotating frames 116a and 116b, from the end portions of which toner transporting flexible sheets made of polyethylene terephthalate radially extend toward the inner surfaces of the first and second chambers 114a and 114b, respectively. As shown in FIG. 15, the two rotating frames 116a and 116b rotate in the same direction indicated by the respective arrows at the same phase angle. It is noted in the conventional toner transporting sheets 117a and 117b that the tip portions of the sheets 117a and 117b always contact the inner surfaces of the chambers 114a and 114b during the operation so that the toner in the each chamber 114a and 114b is strongly pushed out by each sheet 116a and 116b through the first and second openings 115a and 115b into the toner applying chamber 106 and the first toner replenishing chamber 114a.
A magnetism resistance type magnetic sensor 120 is built in the bottom wall of the first toner replenishing chamber 114a for the purpose of detecting the remaining amount of the toner in the first toner replenishing chamber 114a, so that the user can predict the timing of replacement of the developing unit.
However, the conventional developing unit has various disadvantages to be solved.
First of all, the driving torque applied on the first and second toner transporting sheets 117 is quite large since the sheets 117 tend to scoop up a large mount of toner, namely the sheets 117 are so rigid as to not leave the inner surfaces of the first and second chambers 114a and 114b.
Secondly, the flow of the toner through the respective openings 115a and 115b is not smooth because the toner transporting direction, namely the clockwise direction in the drawings, and the direction of the toner flow through the respective openings 115a and 115b are different from each other. In some cases, the toner transported by the first toner transporting sheet will possibly return into the second chamber through the second opening. Still further, since the first and second toner transporting sheets 117 are constantly in contact with the inner surfaces of the chambers 114a and 114b, noise is generated due to the friction between the sheets and the inner surfaces of the chambers. Especially, when the sheets strike the upper walls after they have left the upper ends of the lower walls, a relatively loud noise is generated.
Thirdly, when the developing unit is used under conditions of high temperature and high humidity, the toner in the chambers tends to form a mass or to aggregate due to its moisture absorption so that the fluidity property of the toner is lowered and accordingly the toner image density may become poor.
Finally, the data obtained from the magnetic sensor is not reliable since, even when the amount of the toner in the first toner replenishing chamber is still enough, the magnetic sensor sometimes erroneously generates a signal representing the shortage of the toner in the first chamber. This is caused from the rotational timing of the pair of toner transporting sheets. According to the conventional arrangement as shown in FIG. 15, the rotational angle phases of the first and second sheets 116a and 116b are substantially the same, so that, at a stage where the amount of the toner in the first chamber 114a is reduced, when the first sheet 117 scoops the toner above the magnetic sensor 120, the area above the sensor 120 temporarily becomes vacant of the toner, and then some toner is replenished from the second chamber through the second opening 115a by the rotation of the second sheet 117. Accordingly, a sine curve 201 representing the voltage outputted from the sensor 120 shows a remarkable alternation with a broad amplitude in FIG. 10. Thus, according to the conventional arrangement, even though the first chamber 114a contains enough toner, the voltage represented by the sine curve 201 periodically is lowered below the threshold (S).